How to smoothly move down in Z-index a curved object
Blender comes with an add-on called Mesh:LoopTools. Enable it in the Preferences then you will see it in the N-panel (N).
Select the first 2 vertices at the beginning of the ramp and move them down to the floor along the Z-axis. Then select the vertices of the edge loop that you want to smooth. Make sure the vertice at the beginning is the active one (selected last, see screenshot). After that, use the Space operation a few times with locked X-axis and Y-axis in the options. Count the number of uses for the other side.

Repeat the steps for the other side of the ramp. One or two uses of the Relax operation can smooth out the transition between the ramp and the flat area.
Example: